The goal of this research program is to understand the development of simple cognitive function in neonates. Throughout an organism's life span, there are periods of time when learning is facilitated. This facilitated learning is presumably due to changes in underlying neural structures. The goal of the present proposal is to document the changing characteristics of specific brain areas and their functional significance in learning during infancy. This will extend my previous work using learned infant rat attraction to caregivers as a model system. Normal mother-infant interactions are critical for development and survival of altricial infants. The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. Work during the previous funding period has demonstrated that this optimization includes at least two modulatory systems. First, the noradrenergic locus coeruleus demonstrates unique properties during the first 10 postnatal days of life in the rat that are necessary and sufficient for pups to learn relative odor preferences. Second, the amygdala, a structure involved in fear conditioning in adults, is not activated by aversive stimuli (e.g., odorshock association) until after postnatal day 10, effectively preventing pups from learning relative odor aversions, potentially even to an abusive caregiver. Based on these findings and new preliminary data, it is proposed here that the locus coeruleus and amygdala, combined with the corticosterone stress system function in a unique way during early development to create a sensitive period for infant attachment learning and suppression of learned avoidance behavior that ends during the second postnatal week. The specific aims include: 1) further characterize the role of the amygdala during the sensitive period, including identification of specific nuclei involved in ontogeny of learned (odor-shock) and innate fear, and the ability to extend the sensitive period with amygdala lesions. 2) Characterize the relationship between the locus coeruleus, amygdala and corticosterone during the sensitive period. Pilot data suggest that corticosterone injections can accelerate the ontogeny of learned odor aversions. Finally, 3) determine the role of the locus coeruleus, amygdala and corticosterone in the effects of long-term odor-shock conditioning. Pilot data suggest that odor-shock conditioning beginning during the sensitive period and extending for several days after its normal termination date effectively extends the sensitive period and delays emergence of fear conditioning and amygdala involvement. The results of these studies will be important for our understanding of normal neurobehavioral development and infant attachment behaviors to caregivers.